Chemistry Reference
In-Depth Information
Although air and water tend to dilute and disperse
contamination, soil retains and concentrates many
pollutants by sorption or filtration, preventing their
migration to ground and surface waters. Organic pol-
lutants thus retained then may be metabolised and
broken down by microorganisms. Inorganic and per-
sistent organic pollutants are not degraded and can
concentrate to potentially harmful levels. Pathways
to humans include: take-up into plants and animals,
leading to incorporation in food; leaching into drink-
ing water, leading to inhalation; and suspension in
the atmosphere, leading to direct skin contact.
The UK Department of Environment, Transport
and the Regions (DETR) defines contaminated land
as 'land which represents an actual or potential
hazard to health or the environment as a result of
previous or current use' and many of the most con-
taminated sites are current or former industrial
premises such as mines, smelters, refineries, chemi-
cal manufacturers and scrap yards. The pollutants
may be raw materials, catalysts, electrodes, chemical
products or wastes. Other sites may be contaminated
by leakage or direct application of pesticides, radioac-
tive fall-out, heavy metals from paints, sewage
sludge and vehicle exhausts or pollutant deposits
from aerial discharges.
Where pollution becomes severe the land may be
considered derelict, which is defined as 'so damaged
by industrial or other development that it is inca-
pable of beneficial use without treatment'. It is dif-
ficult to estimate the extent of contaminated land,
not least because of the legal position on liability for
clean-up. The whole of the land surface of the UK is
tainted to some extent by human activities. The area
of contaminated land in Britain, based upon DETR's
definition, has been estimated at 360 000 hectares,
some 208 000 hectares of which have been redevel-
oped. The investigation and remediation costs have
been put at £15.2 billion [14]. Petrol stations, engi-
neering works and coal preparation works are the
three highest cost categories. Sewage works occupy
almost one-third of the total area and chemical
works, with over 4000 sites occupying 7500 hec-
tares, have an estimated remediation cost of almost
£0.5 billion.
Cd, Cr(VI), Hg, Pb and Zn. Other metals, which in
small quantities are essential for healthy growth,
may, when in excess, lead to toxic hazard. These
include Co, Cu, Mn, Mo, Ni and Se.
Most metals are strongly adsorbed by soil,
although Cd and Zn tend to be least strongly
absorbed and can leach under acidic conditions.
Sources of toxic metals include metalliferous
mining, smelting, thermal processing of metals and
wastes, pigments, catalysts, electrodes, polymer
stabilisers, electrical contacts, solders, batteries, se-
wage sludge, domestic waste disposal, corrosion,
food additives, pesticides, wood preservatives, coal
and oil burning, lubricants and shotgun pellets.
Hydrocarbons.
Aliphatic hydrocarbons with
straight and branched chains from CH
4
to C
76
H
154
may derive from all fossil fuels. Single and multiple
ring aromatics with and without nitrogen or sulfur
are important constituents of petroleum deposits.
Hydrocarbons form the main organic pollutants by
weight in soil, especially in industrial sites where
they can pose toxic, flammability and explosion
hazards. The lower molecular weight hydrocarbons
are readily degraded in soil and so pose shorter-lived
hazards than other organic pollutants.
Sources of hydrocarbon pollution include fuel
storage and transportation and leakage and disposal
of used oils.
Persistent organic pollutants and pesticides.
The
PAHs, PCBs, PCDDs and PCDFs are not readily
degraded in the terrestrial environment and their
hazards have been discussed earlier (Section 2.2).
Pesticides comprise a wide range of organic com-
pounds deliberately spread on the land to destroy
insects, mites, nematodes, molluscs, weeds, bacteria
and fungi. Some examples are listed in Table 3.3.
These exhibit a range of toxicities and toxic mecha-
nisms in target species and in non-target species,
including humans. They also exhibit a range of
chemical stability and persistence in the environ-
ment. Generally, the trend has been to reduce risk
by moving from more persistent to less persistent
compounds, albeit at higher cost, as exemplified in
the insecticides where pyrethroids are less persistent
than organophosphates, which in turn are less per-
sistent than organochlorines. However, the break-
down rate may be affected by soil composition and
pH, moisture, microorganisms, temperature and
Major land contaminants
Heavy and toxic metals.
The most important
metals with regard to potential toxic hazard are As,
